In adsorption heat pumps and desiccant air conditioning systems, an adsorbent containing an adsorbate, such as water, is regenerated by heating the adsorbent to desorb the adsorbate, and the regenerated adsorbent is cooled to a temperature at which the adsorbate is adsorbed and is again used to adsorb the adsorbate. An absorption heat pump that utilizes waste heat having a relatively high temperature (120° C. or more) as a heat source for the regeneration of an adsorbent is practically used. However, heat of cooling water from cogeneration units, fuel cells, and automotive engines and solar heat generally has a relatively low temperature of 100° C. or less and cannot be utilized as a heat source for driving practically used absorption heat pumps. Thus, there has been a demand for effective utilization of low-temperature waste heat having a temperature of 100° C. or less or even in the range of 60° C. to 80° C.
Even in adsorption heat pumps having the same principle of operation, the adsorption characteristics required for an adsorbent greatly vary with the available heat source temperature. For example, the waste heat temperature of gas engine cogenerations and polymer electrolyte fuel cells used as high-temperature heat sources ranges from 60° C. to 80° C., and the temperature of cooling water from automotive engines ranges from 85° C. to 90° C. The temperature of a heat source for cooling also depends on the installation location of an apparatus. For example, the temperature of a heat source for cooling is the temperature of a radiator coolant in the case of automobiles or the temperature of water from a cooling tower or river water in the case of buildings and houses. Thus, the operation temperature of an adsorption heat pump installed in buildings ranges from approximately 25° C. to 35° C. on the low-temperature side and approximately 60° C. to 80° C. on the high-temperature side. The operation temperature of an adsorption heat pump installed in automobiles ranges from approximately 30° C. to 40° C. on the low-temperature side and approximately 85° C. to 90° C. on the high-temperature side. Thus, in order to effectively utilize waste heat, it is desirable to develop an apparatus that can operate even at a small temperature difference between the low-temperature heat source and the high-temperature heat source.
In order to satisfactorily operate an apparatus even when the environment surrounding an adsorbent has a relatively high temperature, it is necessary to adsorb an adsorbate at a low relative vapor pressure. In order to reduce the amount of adsorbent and reduce the size of an apparatus, it is necessary to increase the adsorption and desorption amounts of the adsorbent. The desorption of an adsorbate (regeneration of an adsorbent) using a low-temperature heat source requires that the adsorbent should have a low desorption temperature.
Thus, it is desirable that an adsorbent in an adsorption heat pump or a desiccant air conditioning system should
(1) adsorb an adsorbate at a low relative vapor pressure (adsorb an adsorbate at a high temperature),
(2) have high adsorption and desorption amounts, and
(3) desorb the adsorbate at a high relative vapor pressure (desorb the adsorbate at a low temperature).
Existing adsorbents for use in adsorption heat pumps generally include silica gel and zeolites having a low silica/alumina ratio. However, existing adsorbents used in adsorption heat pumps have an insufficient adsorption and desorption capacity when a relatively low temperature heat source is used as a driving source for the adsorption heat pumps. For example, in the water vapor adsorption isotherm of zeolite 13X, which is a representative example of zeolites used in adsorption heat pumps, the amount of water vapor adsorbed on the zeolite is significant at a relative vapor pressure of 0.05 or less and remains unchanged at a relative vapor pressure of more than 0.05. In the regeneration of an adsorbent, the relative humidity of the ambient gas is decreased to desorb adsorbed water. In order to desorb water adsorbed on zeolite 13X, the relative vapor pressure must be decreased using a heat source having a temperature probably in the range of 150° C. to 200° C.
Mesoporous molecular sieves (such as FSM-10) synthesized using a micellar structure of a surfactant as a template are under study as adsorbents for use in heat pumps (Patent Literature 1). Porous aluminum phosphate molecular sieves commonly called AlPO4 are under study as adsorbents for desiccants (Patent Literature 2). In particular, a mesoporous molecular sieve (FSM-10) is a promising material because the difference in adsorption amount of the mesoporous molecular sieve (FSM-10) between relative vapor pressures of 0.20 and 0.35 is as large as 0.25 g/g (Patent Literature 1: graph 4 in FIG. 14; FSM-10). However, the mesoporous molecular sieve (FSM-10) has a small adsorption amount in a relatively low relative vapor pressure range and a small difference in adsorption amount even in a relative vapor pressure range in which the adsorption amount varies greatly, resulting in poor performance of adsorption heat pumps. Furthermore, it has been pointed out that repeated use of the mesoporous molecular sieve (FSM-10) causes deformation of the structure thereof and results in a loss of adsorbent function. Thus, the mesoporous molecular sieve (FSM-10) has a durability problem.
In Patent Literature 3, there is proposed “a zeolite adsorbent for use in adsorption heat pumps that is made of an adsorbent having a relative vapor pressure region in which a change in relative vapor pressure of 0.15 in a relative vapor pressure range of 0.05 or more and 0.30 or less causes a change in the amount of adsorbed water of 0.18 g/g or more in a water vapor adsorption isotherm measured at 25° C.”. Patent Literature 3 discloses that aluminophosphate is preferred. Such a zeolite adsorbent has a greater change in adsorption amount than known silica gel and zeolites in a certain relative vapor pressure range and has a greater dehumidification effect than known silica gel and zeolites of substantially the same weight. In an adsorption heat pump or a desiccant air conditioning system that includes an adsorbent having a great change in adsorption and desorption amounts in a relatively low relative vapor pressure range, because of a great difference in the amount of water adsorbed on and desorbed from the adsorbent, the adsorbent can be regenerated (desorption) at a low temperature, and the adsorption heat pump or the desiccant air conditioning system can be efficiently driven using a lower temperature heat source than before.